1. Field of the Invention
The present invention is generally directed to coatings for plastic and glass articles. In particular, the present invention is directed to phenoxy-based coatings for articles, such as preforms and bottles, having improved resistance to chemicals, abrasion, and heat.
2. Discussion of Related Art
Laminates, such as multilayer preforms and containers having at least one layer of a gas barrier material, i.e., materials having a gas permeability less than that of the substrate, are known. For example, U.S. Pat. No. 5,472,753 to Farha discloses two and three layer laminates, such as preforms and bottles. In the disclosed three layer laminates, the first layer is a phenoxy-type thermoplastic, the second is an amorphous thermoplastic copolyester, and the third is polyethylene terephthalate. In the two-layer laminates, the first layer is a blend of the phyenoxy-type thermoplastic and amorphous thermoplastic copolyester, and the second layer is polyethylene terephthalate (“PET”). The disclosed phenoxy-type thermoplastics include poly(hydroxy ethers), poly(hydroxy ester ethers), and poly(hydroxy amino ethers), and the preferred amorphous thermoplastic polyester is poly(1,4-cyclohexylinemethylene) terephthalate-co-isophthate, which is formed by replacing part of the ethylene glycol and terephthalic acid in PET with cyclohexane dimenthanol and isophthalic acid, respectively. Also, for the purposes of the disclosed invention, PET refers to PET homopolymers as well as copolymers of ethylene terephthate in which up to about 10 mole percent of the ethylene glycol is replaced by other monomeric units, such as diethylene glycol, propane-1,3-diol, butane-1,4-diol, polytetramethylene glycol, polyethylene glycol, polypropylene glycol, 1,4-hydroxymethylcyclohexane, and the like, and up to about 10 mole percent of the terephthalic acid is replaced by monomeric units, such as isophthalic, bibenzoic, naphthalene 1,4- or 2,6-dicarboxylic, adipic, sebacic, decane-1,10-dicarboxylic acids, and the like. For preforms and containers, the inner layer is preferably the PET layer.
U.S. Pat. Nos. 6,312,641, and 6,391,408 and U.S. patent application Ser. No. 10/152,318 to Hutchinson et al. disclose plastic articles, such as bottles and preforms, comprising one or more layers of thermoplastic material, having good gas-barrier characteristics, and methods of making such articles. The disclosed plastic articles are formed from a thermoplastic polyester onto which a barrier layer is applied. The preferred thermoplastic polyester is PET, but other thermoplastic polyesters, i.e., polyethylene 2,6- and 1,5-naphthalate (“PEN”), PETG, polyethylene 1,2-dioxybenzoate, and copolymers of ethylene terephthalate and ethylene isophthalate, are also disclosed. Copolyesters of terephthalic acid, isophthalic acid, and at least one diol are excluded as substrate materials, as those materials are considered barrier materials for the purpose of the invention. The barrier layer is formed from either a copolyester barrier material, i.e., the copolyester of terephthalic acid, isophthalic acid, and at least one diol, and phenoxy-type thermoplastic materials, i.e., hydroxy functional poly(amide ethers), poly(hydroxy amide ethers), hydroxy-functional polyethers, hydroxy-functional poly(ether sulfonamides), poly(hydroxy ester ethers), hydroxy-phenoxyether polymers, and poly(hydroxy amino ethers) (“PHAE”). In addition to disclosing various injection molding techniques, applying barrier coating layers using various dip, spray, and flow coating methods is also disclosed.
As will be recognized by those skilled in the art, the formation of multiple layers on an article may be desirable in some applications, but, no matter what method is used to form the layers, adds complexity and, typically, cost to the manufacture of the article. Therefore, limiting the number of layers in an article, where feasible, may be desirable for economic reasons. As a result, particularly in preforms and containers intended for consumable products, materials, such as gas barriers, that are not approved by the FDA for contact with food preferably perform double duty as the outer, protective layer of the article.
A variety of characteristics are desirable in an outer layer for glass and plastic articles. First, particularly for containers, such as bottles and jars that are subject to the environment of a fill line, the outer layer of the container preferably has good abrasion and chemical resistance, such that the article is resistant to the fill line environment, where a bottle is exposed to physical abrasion, as well as lubricants and beverages or other container contents that may be corrosive. The coating material is preferably also compatible, i.e., miscible, with various colorants, such as pigments and UV blockers, such that, for spray, dip, and flow coating methods, the coating material forms a stable dispersion with the colorant or UV blocker.
In addition, the glass transition temperature, Tg, of the coating on a thermoplastic preform that is intended for blow molding into a hot-fill container should be relatively high, as, during blow molding, the preform is placed in a blow mold having a temperature of about 140° C. As a result, an outer layer having a relatively low Tg, such as a PHAE material, which has a Tg in the range of from about 50° C. to about 70° C., has a tendency to stick to the inner surface of the blow mold. Therefore, with relatively low Tg materials, there is the potential for the production of final products having unacceptable imperfections, damage to the mold, and production line down time.
It has been found that cross-linking the coating material improves its chemical and abrasion resistance. However, one cannot induce a high degree of cross-linking to a preform coating, as a fully cross-linked thermoset material would not stretch during the blow molding step. Subsequently, low levels of cross-linking do not significantly improve chemical and abrasion resistance of the coating. Further, the problems associated with blow molding a relatively low Tg material into a hot-fill container still persist.
In some applications, it may also be desirable to blend an ultraviolet (“UV”) blocker, a pigment, or other additive to the coating. Therefore, as noted above, compatibility between the additive and the coating composition is critical in such applications. However, it has been discovered that in dip, spray, and flow coating methods, such as those disclosed by Hutchinson et al., UV blockers and pigments are incompatible with dispersions of PHAE materials, rapidly precipitating from the dispersion due to insufficient wetting of the additive by the PHAE material.
Therefore, a need exists for a coating composition that has the low gas permeability of a PHAE material, with improved chemical and abrasion resistance, Tg, and compatibility with pigments and UV blockers. The present invention provides such a composition.